1. Field of the Invention
The invention relates to polarization stabilization, more especially to devices and methods for stabilizing with a high accuracy the polarization state of an optical radiation of arbitrary, possibly time variant, polarization.
2. Description of the Related Art
A polarization stabilizer is a device that transforms an input optical beam having an arbitrary input state of polarization (SOP) into an output optical beam with a predetermined SOP and with an optical power, both not dependent on the input SOP. In general, a defined SOP is determined by two parameters: the ellipticity and the polarization azimuth. Such a device is useful, for example, in coherent optical receivers for matching the SOP between the signal and the local oscillator, in fiber optic interferometric sensors, in compensation of polarization mode dispersion of the transmission line and in optical systems with polarization sensitive components. A fundamental requirement is the endlessness in control, meaning that the stabilizer must compensate in a continuous way for the variations of input SOP.
Several polarization control schemes based on finite range components have been presented. In such schemes, in order to achieve an endless control, it is necessary to provide a reset procedure when a component reaches its range limit so that the output SOP does not change during the reset. Generally, reset procedures can be problematic in that they are often associated with complex control algorithms designed to avoid loss of feedback control during the reset.
In polarization division multiplexing (PolDM) transmission at least two channels are launched orthogonally polarized in the optical transmission medium, such as for example an optical transmission fiber. In a typical solution for PolDM transmission, the at least two channels orthogonally polarized are closely spaced, such as for example within 50 GHz spacing or within 25 GHz spacing. In a preferred configuration, the two channels have substantially the same optical wavelength. Typically, while the reciprocal orthogonality of polarization is substantially preserved along the propagation into the transmitting medium, the SOPs of the two channels randomly fluctuate at a given position along the line, such as for example at the receiver section.
In PolDM, a problem arises at the receiving section, or whenever the two orthogonally polarized channels have to be polarization demultiplexed. In general, the polarization demultiplexer is typically a polarization beam splitter, which is apt to split two orthogonal SOPs. In such an application, exact polarization stabilization of the SOPs of the two channels is strongly desired, in order to facilitate polarization demultiplexing. In case of an error in polarization locking, a misalignment occurs between the SOPs of the two channels and those of the demultiplexer. In this case a cross-talk is generated due to an interference between a channel and the small portion of the other non-extinguished channel, which severely degrades the quality of the received signal. For example, in PolDM systems having the individual channels intensity modulated with non-return-to-zero format and directly detected (IM-DD), the penalty to the bit-error-rate becomes about 1 dB for cross-talk of about 20 dB. This means that in case the intensity of the non-extinguished channel is greater than or equal to about 1% of the intensity of the demultiplexed channel, the cross-talk becomes a concern.
Accordingly, in PolDM systems a highly accurate polarization stabilization is needed before polarization demultiplexing. In fact, the cross-talk after polarization demultiplexing is related to the accuracy of polarization stabilization. The accuracy of a polarization stabilizer in terms of optical power may be expressed through a parameter, called uniformity error, defined according to
                              U          =                                                    I                max                            -                              I                min                                                                    I                max                            +                              I                min                                                    ,                            (        1        )            wherein Imax and Imin are the actual maximum and minimum optical intensities, in locked operation, of the polarization-stabilized output radiation when varying the input SOP. In general, the smaller is the uniformity error, the smaller results the cross-talk after demultiplexing. For example, under simplified conditions, a uniformity error of about 1% gives rise to a cross-talk of about 2%.
The article ‘Experimental demonstration of an all-fiber endless polarization controller based on Faraday rotation’ of J. Prat et al., IEEE Photonics Technology Letters 7 (1995) December, No. 12, p. 1430-1432, discloses an all-fiber polarization controller based on the Faraday effect. It consists (FIG. 1 in the article) of a first quarter-wave plate, a first fiber-based Faraday rotator, a second fiber quarter-wave plate and a second fiber-based Faraday rotator.
The Applicant notes that the problem faced by the cited article is transforming a linearly polarized incident light from the local oscillator to a continuously and randomly varying SOP, so as to match that of the received SOP (Section III, first sentence thereof). The proposed scheme and control program accept an uncontrolled fluctuation of the output power at reset with a power reduction up to 50%, which is an unacceptable level of power fluctuation for current applications. The cited article does not address the problem of transforming an input optical beam having an input SOP into an output optical beam with a predetermined SOP and with an optical power, both not dependent on the input SOP.
US patent application 2002/163707 discloses an optical filter wherein an optical input is split into polarization components along separate paths. The polarization components are then fed into an electro-optic device that includes a set of electrodes across which a voltage is applied to adjust a wavelength transmission characteristic of the device.
U.S. Pat. No. 6,823,142 discloses an apparatus comprising an optical divider, first and second dispersion compensators, a switch and a switch controller. The first and second dispersion compensators have, respectively, a first and second polarization converter for converting a polarization of one output light from the optical divider into a linear polarization. Each of the first and second polarization converters comprises a first converter using Faraday rotation, a wave plate, and a second converter for moving the polarization of the output light of the wave plate along the equator of the Poincaré sphere. According to the cited patent, when the switch controller is informed that the driving current of one of the polarization transformer exceeds the limit value, it actuates the switch and set the other polarization transformer to operate in the follow-up priority mode and the first polarization transformer to operate in the restriction priority mode after initializing it (e.g. resetting to zero the driving current to be output).
The Applicant notes that the structure and/or the control algorithm of the apparatus of the cited patent are cumbersome, complex and redundant.
The article ‘Endless tracking polarization controller’ of Kazuhiro Ikeda et al., Furukawa Review April 2003, No. 23, p. 32-38, discloses an endless tracking polarization controller using variable Faraday rotators. It discloses a five-stages (FIG. 5 in the document) or six-stages (FIG. 7 in the document) polarization controller wherein each stage is either a variable Faraday rotator or a variable linear phase shifter (i.e. a linearly birefringent element) obtained by the arrangement of a variable Faraday rotator sandwiched between two quarter wave plates orthogonally oriented. The proposed scheme is complex and the described control algorithm, which includes a reset procedure, is cumbersome and complicate.
U.S. patent application 2003/0122063 discloses a polarization transformer operable to reorient polarization components of an incident optical signal. The polarization transformer includes a continuously adjustable retarder and a limited-range adjustable retarder. The cited patent discloses implementation of continuously adjustable retarder by way of conventional waveplates, lithium niobate devices, semiconductor devices and liquid crystal devices such as vertically-aligned nematic liquid crystal cells using variable lateral electric fields. Different technologies are disclosed to construct limited-range adjustable retarder including: liquid crystal cells, lithium niobate crystals, PLZT materials and mechanically or thermally stressed fiber. The Applicant notes that the cited patent application only discloses variable birefringent elements which are based on the electro-optic or elasto-optic effect, having the disadvantages described further below. Also, the use of a continuously adjustable retarder poses severe limitations on the choice of a variable retarder suitable for this purpose. In addition, the Applicant believes that, even thought the control algorithm is not described in details, the proposed scheme does not allow a simple control algorithm.
WO03/014811 patent discloses an endless polarization stabilizer based on one or two pairs of birefringent components that each have fixed eigenaxes and variable phase retardation, as well as an endless polarization stabilizing method based on a simple feedback control algorithm. A single-stage configuration based on two linearly birefringent variable retarders with finite birefringence range and respective eigenaxes oriented at approximately ±45 degrees relative to each other is described. The endlessness is obtained by commuting the phase retardation of one retarder, when the retardation of the other retarder reaches a range limit. It is also described a single-stage configuration based on a variable linear retarder and a variable polarization rotator.
Furthermore, in WO03/014811 patent application a two-stage configuration has been presented wherein the two stages are controlled independently by respective algorithms that are similar to that used for the single stage configuration. Each stage comprises two variable retarders with finite birefringence range and respective eigenaxes oriented at approximately ±45 degrees relative to each other.
The Applicant has found that none of the known solutions for polarization stabilization is at the same time accurate enough to meet current applications' specifications, especially in the context of PolDM transmission, and simple enough to be practically feasible and operable.
The Applicant has noted that by employing variable birefringent elements which are based on the electro-optic or elasto-optic effect, such as the variable linear retarder of patent application WO03/014811, the resulting polarization stabilizer exhibits an accuracy of polarization tracking which is highly sensitive to possible errors of orientation of the variable birefringent elements. In real word, and especially in a context of industrial production of the stabilizer, the unavoidable errors in the orientation of the variable elements are sufficient to jeopardize the functionality of the device.
In particular, in a variable birefringent plate having fixed eigenaxes, based on the electro-optic or elasto-optic effect, the direction of the variable electrical field or, respectively, the variable strain field and the direction of propagation of the optical beam must be suitably oriented with respect to the internal structure of the plate. An error in this orientation causes a rotation of the birefringence axes in correspondence to the variation of the field intensity. This in turn critically degrades the performances of the polarization stabilizer based on these elements, for example it increases the uniformity error for the optical intensity of the SOP-stabilized output radiation.